The present invention relates in general to battery protection and, more particularly, to a circuit and method of sampling the status of serially coupled battery cells.
Batteries are used in a wide variety of applications including digital pagers, cellular telephones and notebook computers. Lithium ion and polymer batteries are often selected for such applications because of their rechargeable characteristics, low weight and high energy density. A lithium ion battery cell typically provides a voltage of about 4.2 volts. Since many applications require 12 to 15 volts of operating potential, several battery cells may be serially coupled together in a battery pack to provide the needed operating potential. Once the batteries have been discharged by shelf life or during normal usage in an application, it is necessary to recharge the battery pack. A common problem experienced in many lithium ion and polymer battery applications occurs during the re-charge process where it is possible to over-charge one cell if another cell is mismatched or defective. The over-charging can cause catastrophic failures with potential safety problems for the end user. Moreover, it is possible for one or more of the serially coupled battery cells to be over-discharged which tends to reduce the capacity of the battery cells and battery pack. Both over-voltage and under-voltage conditions should be avoided.
In the prior art, battery protection and monitoring schemes have been developed that monitor the battery cells to determine under-charge or over-charge conditions. Most if not all battery protection circuits involve continuous monitoring all battery cells simultaneously to determine their status. The continuous monitoring consumes excessive power which is typically supplied by the battery pack. The prior art battery protection circuit thus reduces the shelf life and operating time between charges.
Hence, a need exists to monitor the individual cells of the battery pack without undue drain on the battery pack.